As a method for synthesizing alcohols, Guerbet reactions that synthesize alcohols dimerized by condensing two molecules of a raw material alcohol are known (see patent documents 1 to 12 (PTLs 1 to 12)). Many of these Guerbet reactions are a liquid phase reaction and are performed at normal pressure or in a pressurized state. In addition, in the case of performing the reactions in a gas phase, the reactions are normally performed at normal pressure. For example, patent document 8 discloses a reaction comprising bringing two or more raw material alcohol gases into contact with a hydroxyapatite (calcium hydroxyapatite) at normal pressure (paragraph [0036]), and patent document 12 discloses a reaction comprising bringing ethanol gas into contact with strontium phosphate apatite (strontium hydroxyapatite) at normal pressure (paragraph [0025]).